The oxide ceramics possess high-temperature strength and are excellent in heat resistance, oxidation resistance and corrosion resistance. Such oxide ceramics can reliably be used up to at least a temperature lower by not higher than several hundred degrees centigrade from a melting point thereof. Therefore, rare earth oxides (oxide of rare earth element or a mixture thereof) and alumina are expected to be used as a high-temperature ceramic material. Particularly, a mixed ceramic containing two oxides is considered to be effective as a so-called high-temperature material because the melting point is about 2000.degree. C.
In the mixed oxide ceramics, however, when a mixture of oxides is fired to obtain a sintered body, abnormal growth of crystal grains causes a large crystal grain size of not less than 100 .mu.m, so that there are caused pores which reduce densification. Furthermore, the resulting sintered body is weak in strength, toughness and hardness.
For instance, in case of Ln.sub.4 Al.sub.2 O.sub.3 compounds, Ln is Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and a mixture thereof, and LnAlO.sub.3 compounds, Ln is Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and a mixture thereof, there is a bad drawback that very brittle polycrystalline sintered bodies are only obtained because a twin is formed through martensitic transformation in the sintering.
On the other hand, one of the methods for controlling the abnormal grain growth in the polycrystalline sintered body of the above mixed oxide ceramics is to use a method for controlling an addition of a third substance.
It is an object of the invention to repeatedly establish the above control technique by adding a third substance to properly control the crystal grain size of the sintered body.